Crystal structure of 2-methyl-1H-imidazol-3-ium 3,5-dicarboxybenzoate

The structure of a 2-methyl-1H-imidazol-3-ium trimesate compound was determined by single-crystal X-ray diffraction. The compound is composed of protonated 2-methylimidazole and singly deprotonated trimesic acid molecules.


Chemical context
Trimesic acid, also known as 1,3,5-benzenetricarboxylic acid (Hbtc), and 2-methylimidazole (mIm) are two well-known organic compounds with significant applications in various industries.For example, mIm, a nitrogen-containing heterocyclic organic compound, serves as a versatile chemical intermediate that is used extensively in the synthesis of pharmaceuticals, photographic and photothermographic chemicals, dyes and pigments, agricultural chemicals, and in rubber production (Hachuła et al., 2010;Chan, 2004).On the other hand, Hbtc is a planar and highly symmetrical trifunctional compound, which finds use in coating materials, adhesives, plastics, and even in the pharmaceutical industry for drugs and gene carriers.Notably, some dendrimers based on Hbtc have been employed as biomolecular delivery systems (Salamon ´czyk, 2011;Mat Yusuf et al., 2017).Both Hbtc and mIm are also well-established ligands frequently employed in the synthesis of metal-organic frameworks (MOFs).For example, mIm is used in the synthesis of ZIF-8 (zeolitic imidazolate framework À 8; Park et al., 2006), while Hbtc is employed in the production of HKUST-1 (Hong Kong University of Science and Technology À 1; Chui et al., 1999).

Structural commentary
Compound 1 crystallizes with one singly deprotonated trimesate (btc) molecule and one 2-methyl-1H-imidazol-3-ium (HmIm) molecule in the asymmetric unit, space group C2/c.An ellipsoid plot illustrating these molecules can be seen in Fig. 1.The hydrogen atoms attached to O2 and O3 lie in close vicinity to an inversion center or twofold axis, respectively, and as a consequence, each is disordered between two neighboring molecules with equal occupancy.
Table 1 exhibits selected bond distances and angles of the btc ion.Among these bonds, the shortest non-hydrogen bond occurs between C9 and O6 with 1.224 (2) A ˚, while the largest is between C1 and C7 with 1.511 (2) A ˚.The O-C and C-C bond lengths are in the ranges 1.224 (2)-1.320(2) A ˚and 1.388 (2)-1.511(2) A ˚, respectively.These distances are slightly larger than those corresponding to the reported Hbtc molecule (Tothadi et al., 2020), which range between 1.229 (5) and 1.303 (5) A ˚for the O-C bond distances and between 1.381 (6) and 1.494 (9) A ˚for the C-C bond distances.Additionally, Hbtc exhibits O-C-O angles of the carboxylate group ranging from 124.4 (4) to 125.0 (4) � and C-C-C angles of the aromatic ring ranging from 119.0 (4) to 121.1 (4) � , while btc shows slightly wider ranges with O-C-O falling in the 123.9 (2)-126.1 (2) � range and C-C-C angles in the 118.9 (2)-121.4(4) � range.
The main difference between the Hbtc molecule of Tothadi and co-workers and the btc ion within the present compound lies in their torsion angles.In the Hbtc molecule, the oxygen atoms are nearly coplanar with the aromatic ring, with torsion angles deviating from 0 or 180 � by no more than 4.2 (4) � .In contrast, the btc ion in compound 1 shows a wider deviation range, spanning from 4.2 (2) to 16.6 (2) � .Oxygen atoms O3 and O4 in 1 are the most coplanar with the aromatic ring, as illustrated by the torsion angles O3-C8-C3-C2 and O4-C8-C3-C4 of 4.4 (2) and 5.7 (3) � , respectively.The difference between Hbtc and btc is further highlighted through a molecular overlay (Fig. 2) generated by the Mercury software (Macrae et al., 2020).The root-mean-squared deviation (r.m.s.d.), as calculated by Mercury is 0.1356 A ˚, with the major distinction being in the positions of atoms O5 and O6 (Fig. 2a).The molecular structure of 1 with displacement ellipsoids drawn at the 50% probability level.

Table 1
Selected bond lengths (A ˚), bond angles ( � ), and torsion angles ( � ) of the btc ion.Selected bond distances and angles for the mIm ion are presented in Table 2.The C-C bond distances are 1.345 (3) and 1.481 (3) A ˚, whereas the N-C distances range from 1.327 (2) to 1.377 (2) A ˚.These distances are slightly shorter than those found in the neutral mIm molecule reported by Hachuła et al. (2010), where the C-C bond distances are 1.367 (1) and 1.488 (1) A ˚, and the N-C distances range from 1.329 (1) to 1.385 (1) A ˚.It is worth noting that imidazole derivatives often exhibit an asymmetry in the two endocyclic N-C bonds (Hachuła et al., 2010), a characteristic also observed in compound 1, where N1-C12 [1.326 (2) A ˚] shows greater double-bond character than N2-C12 [1.335 (2) A ˚].However, this difference is more pronounced in the neutral molecule [0.022 (1) A ˚] compared with the HmIm ion in 1 [0.008 (3) A ˚], possibly due to the protonation in the HmIm ion.
Compared with the neutral mIm molecule, protonation in the HmIm ion results in a more symmetrical heterocyclic ring.This increase in the symmetry is observed in the C-C-N and N-C-N angles of the heterocyclic ring, which closely approach the ideal pentagon angle of 108 � in the HmIm ion, with a maximum deviation of 1.6 (2) � , while in the neutral mIm molecule, this deviation is slightly larger, at 3.4 (1) � .However, in both cases the carbon of the methyl group is almost coplanar with the heterocycle ring as observed in the torsion angles C10-N1-C12-C13 and C11-N2-C12-C13 of À 179.5 (2) and 179.6 (2) � for HmIm and À 179.4 (1) and 179.3 (1) for mIm.
Fig. 2b illustrates the molecular overlay between the HmIm ion in compound 1 and the neutral mIm molecule as reported by Hachuła and co-workers.The figure demonstrates that contrary to the btc ion, the HmIm ion bears a closer resem-blance to its neutral counterpart.This similarity is further supported by the r.m.s.d.value calculated by Mercury, which has a value of 0.0320 A ˚.

Supramolecular features
The crystal packing in 1 is primarily based on hydrogen bonds and �-� interactions.Table 3 provides a summary of the hydrogen bonds found within the compound.Hydrogen atoms H2 and H3 are involved in an infinite chain of hydrogen bonds.
As a result of the symmetry of the crystal, and the negative charge of the trimesate anion, the protons H2 and H3 have an occupancy of only 50%, meaning that in the asymmetric unit, the negative charge is distributed evenly between the two carboxylates.In other words, if O3 is protonated, O2 from the same molecule is not and the neighboring trimesate molecules participating in the hydrogen-bonded chain will have O2 protonated and O3 not (Fig. 3).As illustrated in Fig. 4a, hydrogen bonds N1-H1� � �O1, N2-H2B� � �O6, and O3-H1� � �O3 form undulating chains that extend along the [302] direction, while �-� interactions [centroid-centroid distance of 3.770 (2) A ˚], both among mIm and between btc The two possible mutual positions of the hydrogen atoms H2 and H3 (orange or violet) and the resulting hydrogen bonds in the infinite chain of the trimesate anions.ions, stack the chains along the b-axis direction (Fig. 4b).

Database survey
A search for the title compound in the Cambridge Structural Database (CSD, Version 5.43, update of November 2022; Groom et al., 2016) did not match with any reported structures.

Synthesis and crystallization
In a typical synthesis, solutions of CoCl 2 •6H 2 O (2.5 ml, 0.02 M), mIm (65 ml, 1.58 M) and btc (500 ml, 0.12 M) were mixed without stirring.Within less than a minute, a blue precipitate was formed.The resulting heterogeneous mixture was allowed to slowly air-dry.After complete solvent evaporation, we obtained a mixture of the title compound, the previously reported cobalt complex 2, and an unidentified phase.
Although the blocky colorless crystals of the title compound can be easily identified in the mixture, all attempts to separate them from the other components by other than mechanical means were unsuccessful.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4.The positions of hydrogen atoms were refined with U iso (H) = 1.2U eq (C or N) for CH and NH groups and U iso (H) = 1.5U eq (C or O) for others.Hydrogen atoms H2 and H3, each lying close to a symmetry element, were refined with a fixed occupancy of 0.5.The protons of the methyl group were refined as disordered over two geometrically idealized positions.The most disagreeable reflection (002) with an error/s.u. of more than 10 was omitted using the OMIT instruction in SHELXL (Sheldrick, 2015b).H atoms treated by a mixture of independent and constrained refinement �� max , �� min (e A ˚À 3 ) 0.24, À 0.28 Computer programs: APEX2 and SAINT (Bruker, 2016), SHELXT2018/2 (Sheldrick, 2015a), SHELXL2018/3 (Sheldrick, 2015b), and OLEX2 (Dolomanov et al., 2009).

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes. Fractional Figure 1

Figure 4
Figure 4 Crystal packing in compound 1.(a) View down the b axis showing undulating chains formed by HmIm and btc ions through hydrogen bonding (blue lines), (b) view along the [101] direction illustrating the stacking of the chains via �-� interactions (green lines), and (c) view of the interconnection of chains in an out-of-phase arrangement.

Table 4
Experimental details.